1. The Field of the Invention
The present invention relates to devices operative to be coupled to high voltage electric power lines.
2. The Relevant Technology
Typically when it is necessary for a power company to transmit electrical power over long distances, it is transmitted at relatively high voltages. These high voltages are often much higher than the voltages used by customers of the electric power. When a voltage greater than 1 kilovolt (kV) and less than 40 kV is used for a particular power line, the power line is typically referred to as a distribution line. When a voltage greater than 40 kV is used, the power line is typically referred to as a transmission line. Transmission lines are generally used to transmit larger amounts of power over greater distances than distribution lines.
When a producer of electric power wants to connect to the electric grid, a connection can be made at either distribution line or transmission line level depending on the capacity of the producer's generating plant. Increasingly, due to deregulation in the power industry, the producer's generating plant is not owned by the same company as the transmission and distribution lines the plant will connect to. These types of producers are often referred to as independent power producers (IPPs). Since the power lines and the generating plant are not owned by the same company, it becomes much more important to accurately determine the amount of power the plant is feeding into the electrical grid through the transmission or distribution lines. Even when the generating plant and the power lines are owned by the same company, it is often advantageous to accurately monitor the amount of power being fed into the electrical grid.
IPPs will often only produce power when the demand for power is such that it is economical to do so. Therefore some IPPs may only produce power for a small percentage of time during a year. When the IPP is not feeding power into the grid, its generators are normally shut down and the IPP actually draws power from the electrical grid. The amount of power drawn from the grid in this situation is usually much smaller than when the IPP is generating power. A power usage by the IPP that is only 1/1000th of its power generation capability or less is quite possible when the plant is idle. It is often desirable that the IPP be accurately billed for the energy it consumes during idle periods and accurately compensated for energy generated during active periods.
It is desirable for the energy meter and other instrumentation monitoring the flow of power to and from the producer to accurately measure both the power usage when the producer is idle and the power production when the producer is operating. This means that accurate energy metering and monitoring over a wide dynamic range such as 1000 times may be necessary. The energy metering and monitoring is often done at grid level voltages. Therefore, the voltage does not vary greatly (perhaps by +/−10% of its nominal value). This means that the wide variation in power flow seen by the energy metering and monitoring equipment is primarily due to the variation in current.
An energy meter capable of measuring over a wide dynamic range of current is described in U.S. patent application Ser. No. 10/341,079 to Hyatt et al. and entitled “Energy Device with an Extended Dynamic Range on Current Readings” which is incorporated herein by reference.
Using an energy meter with a wide dynamic range capability for current is part of the solution for accurately monitoring the flow of power to and from a producer. Energy meters for this type of application are typically connected through external current and voltage sensors. At least the current sensors themselves should also have a wide dynamic range. Optical current sensors such as those described in the document entitled “OPTICAL TECHNOLOGY: A NEW GENERATION OF INSTRUMENT TRANSFORMER” by Klimek published in Issue 2/2003 of Electricity Today have often provided the largest dynamic range. These sensors are often mounted on large insulator stacks and weigh 100 s of kilograms.
The installation costs for sensors mounted to high voltage transmission lines may often be significant. In fact, the installation cost may be more than the cost of the sensor itself in some cases. Some of the reasons for this include the large size and weight of the sensors and the downtime that is experienced when installing, re-installing or replacing a defective sensor. Most of the weight and size of many of these sensors is the insulator used to isolate the sensor from ground and support the sensor.
Another consideration that must be taken into account when accurately accounting for energy produced and consumed by a producer is that the instrumentation may have to be regularly calibrated to ensure accuracy. This means that the sensors may be regularly un-installed and sent for calibration while a replacement sensor is installed. This results in the install/re-install costs as well as significant shipping costs due to the weight of the sensors. This recalibration interval may be approximately every three years and the install-reinstall costs for a single sensor may be in the neighborhood of $100,000 US.
Sensors that power themselves using the magnetic field generated by the current flowing through the line they are monitoring are also available. One such device is described in U.S. Pat. No. 4,799,005 to Fernandes entitled “Electrical Power Line Parameter Measurement Apparatus and Systems, Including Compact, Line-Mounted Modules”. Although this device may enable decreased install-reinstall costs, but because it is powered from the magnetic field generated by the current flowing through the line it is measuring, it may not be usable when the line current varies over a wide dynamic range. This is due to the fact that the magnetic field generated at low current may not be adequate to generate enough power to power the device or the current transformer (“CT”) and associated circuitry used to power the device may be too complex or expensive to be practical.
High voltage electric power lines criss-cross the landscape. These lines pass over waterways, valleys, highways, through and around cities, etc. They are sometimes visible to observers on the ground. They are sometimes not particularly visible to aircraft. This is especially a problem where they cross vast expanses such as over valleys or waterways where there is a long distance between support towers. Transmission line support structures are often illuminated with obstruction lights through the employment of low voltage AC mains distribution power supply means and standard red incandescent obstruction light fixtures as specified by the Federal Aviation Advisory Circular 150/5345-43E. Identification of the actual transmission line catenary wires is often limited to the suspension of passive, brightly painted spheres. These afford little aeronautical identification at nighttime or under conditions of reduced visibility.
Recent technical advances have resulted in the ability to attach obstruction lighting directly to high voltage transmission line wires through a number of self-powering means not requiring connection to an external power source. Federal Aviation Advisory Circular AC 70/7460-1K now provides guidelines detailing the use of direct catenary wire obstruction lighting. U.S. Pat. No. 5,448,138 entitled “Aeronautical obstruction light” describes a device capable of direct obstruction illumination that extracts power through magnetic coupling using a coupling coil mounted in proximity of the power line. P and R Technologies of Beaverton, Oreg., offer a number of self-powered transmission line obstruction markers. Their SpanFlash™ series of transmission line markers employ a magnetic field power supply that requires a minimum of 50 Amperes for correct operation combined with a gas discharge lighting solution. This technique does not work when transmission line currents fall below a lower limit that results in insufficient magnetic field to support adequate power generation. Many transmission lines, particularly those coming from independent power producers, experience wide operating current ranges depending on load conditions.